OK, I read through lots of this forum trying to find out some info as to when should you fill your tank back up. I have a 65 cu.ft tank 4500 psi/ How low should I let the psi go down to, before I go get it filled again and the reasoning why. A fired of mine told me he fills his when it goes down from 4500 to 3000. I sent him a email asking why but he is busy so I put the question up here. maybe common sense tells me that letting the air too low it will start to have moisture problems?? I was going to run it down till it emptied, but now I need to ask. My fills cost me 6 bucks, no matter what the gauge is at when I bring it in to have filled